Different Crystal Form Of Calcium Sulphate Crystal Growth Control

The calcium sulfate crystal is a new-type material which has the fine function, the low price, the good radios between property and price, the wider application fields and the broader market foreground. Controlling the growth condition of the calcium sulfate crystal, we can prepare the calcium sulfate crystal with the different shapes, such as whisker, sheet-shaped and spindle-shaped crystal etc.This paper used the synthetic calcium sulfate as the precursor to prepare the sheet-shaped and spindle-shaped calcium sulfate crystal by the hydrothermal method, and investigated systematically the effect of the reactant quantity and concentration of synthetizing precursor and the temperature, time and pH of the hydrothermal reaction to the calcium sulfate crystal preparation. Based on it, we had accomplished the calcium sulfate crystal preparation process research. The research obtained:(1) The sheet-shaped and the spindle-shaped calcium sulfate crystal preparation:①30 mL 0.6 mol/L sodium sulfate solution and 25 mL 0.6 mol/L calcium chloride solution reacted to prepare the precursor, and got the sheet-shape calcium sulfate crystals with a mean length of 405～414μm and the mean aspect ratio of 3.5～3.8 when the hydrothermal treatment is 4 h at pH=5.29 and the 180℃hydrothermal temperature;②25 mL 0.6 mol/L sodium sulfate solution and 35 mL 0.6 mol/L calcium chloride solution reacted to prepare the precursor, and got the spindle-shaped calcium sulfate crystals with a mean length of 750-813μm and the mean aspect ratio of 11.1～11.7 when the hydrothermal treatment is 4 h at pH=4.74 and the 200℃hydrothermal temperature;(2) The growth process kinetics model of the calcium sulfate crystal:We analyzed the growth processes for whisker, the sheet-shape and the spindle-shape calcium sulfate crystal, their growth process may be divided into five stages, namely the precursor following precipitation—dissolution—whisker crystallization—crystal shape transform—the new crystal shape crystallization, but the whisker growth process had only the front of three stages. Because the growth change for the front three stages of the five stages had the greater effect to the object crystal shape, we had conducted the kinetics simulation for these three stages and obtained respectively the models:①The kinetics model for the calcium sulfate whisker:the first stage model was:-(dc)/(dt)=-0.0002+1.2092(c0-c)the second stage model was: (dc)/(dt)=0.1296cthe third stage model was:-(dc)/(dt)=0.0331-1.2806(c0-c)②The kinetics model for the sheet-shaped calcium sulfate crystal:the first stage model was:-(dc)/(dt)=0.0237-3.0338(c0-c)the second stage model was: (dc)/(dt)=2.1230c-0.01740the third stage model was:-(dc)/(dt)=0.0885(c0-c)0.2082③The kinetics model for the spindled shape calcium sulfate crystal:the first stage model was:-(dc)/(dt)=0.0505-0.7402(c0-c)the second stage model was: (dc)/(dt)=1.0312cthe third stage model was:-(dc)/(dt)=0.2408(c0-c)0.5470 The first stage was the polynuclear surface reaction controlling, the second stage was main the first-order reaction process which the calcium sulfate dissolves, the third stage is surface reaction controlling.